Sasuke could be the cause. If Sasuke were to betray Naruto again in the fanfiction, it would surely make Naruto pissed off. Their relationship is complex and Sasuke's actions have a big impact on Naruto.
A new and powerful villain who mocks Naruto's dreams and ideals could be the cause. Naruto is very passionate about his dreams of becoming Hokage and bringing peace. If someone were to belittle those dreams, it would make him extremely angry.
It could be Orochimaru. If Orochimaru were to show up and start causing trouble in the Hidden Leaf Village once more, Naruto would get pissed off. Orochimaru has a history of doing evil things, and Naruto wouldn't tolerate him threatening his home and friends.
A new, powerful and arrogant villain introduced in the fanfic could be the cause. This villain could constantly mock Naruto and his ideals, and try to undermine his efforts to protect the village. Such blatant disrespect would make Naruto really pissed off as he is very protective of his home and the values he stands for.
Sasuke could be a possible cause. If Sasuke were to go back on his word about protecting the village or if he were to harm their friends again, Naruto would be pissed. Their relationship is complex, and Sasuke's actions often have a strong impact on Naruto.
Sasuke could make Naruto pissed off if he goes back to his old ways of seeking power and revenge. Naruto has worked hard to bring Sasuke back to the right path, and if Sasuke strays again, it would surely anger Naruto.
Maybe Naruto gets pissed off because his friends don't believe in him. For example, if they suspect him of being involved in something bad without evidence.
One possible scenario could be that Naruto gets pissed off when he discovers someone has been spreading false rumors about his friends in the fanfiction. This would really push his buttons as he values his friends highly.
One common theme is revenge. When Naruto is pissed off, often he's seeking revenge against someone who has wronged him or his friends. For example, if Orochimaru did something to harm Sakura, Naruto would be pissed off and his revenge - driven actions would be the focus. Another theme could be self - discovery. His anger might lead him to discover new powers or a new side of himself. Like his anger might unlock a hidden bloodline limit that he didn't know about before.
Perhaps an outside force that manipulates the Young Justice team into going against Naruto. This could be a villain who wants to create chaos in Naruto's world. The Young Justice, being unaware of the manipulation, start acting in ways that piss Naruto off. In this case, the real antagonist is the behind - the - scenes villain, but the Young Justice members end up being the ones Naruto has to deal with directly, making them sort of antagonists in Naruto's perspective.
Well, perhaps in this fanfiction, Naruto is somehow involved with the Young Justice team. Maybe they accidentally undermine one of Naruto's important missions or goals. For example, they could interfere in a battle where Naruto was about to achieve something crucial, like rescuing a friend or stopping a powerful enemy. This would surely piss Naruto off as he's very passionate about his friends and his village.
In a 'naruto gets pissed off fanfic', one possible scenario could be that Naruto discovers someone has been slandering his friends behind their backs. He would get pissed off and confront the person directly, his eyes blazing with anger.
One reason could be betrayal. If someone he trusted, like a fellow ninja or an old acquaintance, turned out to be working for his enemies, that would surely make him pissed. It's a huge blow to his trust, which is a big part of his character.